1. Field of the Invention
This invention relates generally to the field of munitions propellants and in particular it relates to propellants that are functionally-graded over their cross-sectional areas and corresponding method of manufacture in which individual grains of the munitions propellant have particle concentration, and hence burn rate distributions, including a fast burning core and slower burning outer region(s).
2. Description of Related Art
Recently, significant interest has been expressed in the use of “fast core” and/or “plateau burning” munitions propellants as one way to improve the performance of munitions. At present, such fast core propellants are propellants prepared using two separate propellant formulations. A first, fast burning propellant formulation forms the center or “core” of individual propellant grains, and a second, slower burning propellant formulation forms the outer layer(s) of the individual propellant grains. When constructed in this manner, the outer layers burn more slowly than the faster burning core.
The construction of such propellants is facilitated by the knowledge that the changes in concentration of energetic particles that comprise energetic propellants will change the burn rate. For example, the experimental work of B. Homan, D. Devynck, P. Kaste, R. Lieb, D. Bullock and A. Juhasz that was described in a paper entitled “BNMO/NMMO/RDX TPE Propellant Performance as a function of nitramine particle size and solids loading”; published in the US Army Research Laboratory Report, ARL-TR-2624 in December 2001 disclosed that the burn rate of an energetic suspension is a function of the concentration of energetic particles and their size(s). FIG. 1 shows the selected results of this study wherein the concentration of RDX particles increases the burn rate across the entire range of particle sizes considered in the 2 to 32 micron size range.
In an attempt to explore the improvement(s) and thereby improve the ballistic performance of guns, multi-layer munitions propellants having a fast burning propellant formulation sandwiched between slower burning propellant formulations (cross-sectional, functionally graded propellants) have been prepared. Unfortunately, while these munitions propellant “laminates” are based upon sound theory and offer much promise, they have proven extremely difficult and quite costly to make.
Accordingly, new methods that facilitate and/or permit the preparation of a cross-sectional, functionally graded munitions propellants exhibiting a continuum of burn characteristics—i.e., from slow to fast—are highly desirable and if provided in a reproducible, cost effective and convenient manner—would represent great progress in the art.
Seemingly unrelated studies in the field of rocket motor design and construction as well as other theoretical/experimental undertakings has led to certain knowledge of solid phase migration that takes place during processing of solid rocket motor propellants. In particular, it was experimentally observed that particles in a concentrated suspension subjected to inhomogeneous shear fields rapidly migrate away from regions of high shear rate in inhomogeneous shear fields and develop anisotropic particle structures. More specifically, in a bimodal suspension such as a rocket motor propellant, coarse fraction(s) migrate much faster than the fine fraction(s), which leads to size segregation of initially, well-mixed suspensions. (See, e.g., Alan L. Graham, “PROCESSING-INDUCED MIGRATION, SIZE SEGREGATION, AND STRUCTURE FORMATION IN SOLID PROPELLANTS”, PL-TR-92-3013; Final Report; Los Alamos National Laboratory, Los Alamos N. Mex. 87545; April 1992.) In view of its decidedly undesirable nature in the context of solid rocket motors, the phenomenon was appropriately named, “demixing”.
Despite its undesirable effect on the manufacture of solid rocket motors, we have nevertheless advantageously applied this demixing phenomenon to the design and manufacture of new types of i.e., munitions and propellants, which are functionally graded over their cross-sections and are the subject of the present invention.